1. Field of the Invention
This invention relates to rheology-modified thermoplastic polyolefins, processes for making rheology-modified thermoplastic polyolefins and processes for shaping them into molded articles. In particular, this invention relates to rheology-modification of ethylene interpolymers such as ethylene/.alpha.-olefin polymers.
2. Description of Related Art
Polymers and numerous additives are typically compounded into formulations which are then totally cross-linked for enhanced strength properties of the finished article. The starting polymer, prior to cross-linking, must have adequate performance properties such that it may be formulated or compounded with various additives and still maintain processability. For example, in a wire and cable coating operation, the composition must have "green strength", also known as "melt strength", to remain on the wire after coating, and not sag or deform on the wire until the composition is cured. Otherwise the wire will have thin spots and the insulating value of the composition is lost. The composition must also undergo a final cure step and achieve good physical properties, such as tensile strength, elongation, and 100% modulus (stress at 100% strain). Typical curing occurs through use of peroxide or irradiation, and for polyethylene in general, the curing through crosslinking phenomenon is well documented (see, for example, Radiation Effects in Materials, A. Charlesby, editor, Pergamon Press, 1960). Polyethylene, especially heterogeneous linear low density polyethylene (LLDPE), when exposed to peroxide and/or radiation under proper conditions, forms gels as the molecular weight builds.
Usually the polymer selected to compatibilize all of the various components used in wire and cable coating operations is an elastomer such as ethylene/propylene rubber (EPR) or ethylene/propylene diene monomer terpolymer (EPDM). These types of very low density polymers (i.e., polymers typically having a density less than 0.92 g/cm.sup.3) are relatively expensive (as compared with traditional linear low density polyethylene polymers) and contain a very high percentage by weight of comonomer(s) (e.g., propylene, dienes). Lowering the density of the polymer also increases the ability of the polymer to hold more filler and oil.
There have been a few recent announcements regarding new polymers which are said to be effective substitutes for EPR and EPDM. Union Carbide Chemicals and Plastics, Inc., announced in 1990 that they have developed a new cost effective class of polyolefins trademarked Flexomer.TM. Polyolefins that could replace expensive EPR or EPDM rubbers. These new polyolefins are said to have bridged the gap between rubbers and polyethylene, having moduli between the two ranges.
While the development of new lower modulus polymers such as Flexomer.TM. Polyolefins by Union Carbide or Exact.TM. polymers by Exxon has aided the elastomeric formulation marketplace, there continues to be a need for other more advanced, cost-effective polymers which can ultimately be fully cross-linked to form a polymer aggregate such that the bulk polymer is a covalently bonded network of polymer chains, but which also have good physical properties and processability prior such to complete cross-linking.
Others have attempted to modify polyolefins in various ways to try to achieve such goals. For example, in Chemical Modification of Linear Low Density Polyethylene, by T. K. Su, R. G. Shaw, P. J. Canterino, E. A. Colombo and T. H. Kwack, published in ANTEC '87 SPE Technical Papers, vol. 33, pp. 1271-1275, linear low density polyethylene (LLDPE) was crosslinked using peroxide free-radical initiation. This modification is said to result in chemically modified LLDPE without creating gels. However, Su et al. also report that peroxide modification of LLDPE results in higher apparent viscosity throughout the range of shear rate (see FIG. 2 of Su et al.). This change in viscosity indicates growing molecular weight as a result of the peroxide modification and results in modified LLDPE which does not have the same processability as the unmodified LLDPE, especially in the high shear range.
In PCT/GB85/00142 (published as WO 85/04664) ("PCT '142" herein), LLDPE is treated to enhance the polymers' suitability for extrusion conversion into hollow articles (e.g., tubes, sheathing, and wire and cable insulators). PCT '142 states that treating LLDPE having a melt index over 3 g/10 minutes with "moderate quantities of peroxide does not bring about an adequate broadening of molecular weight distribution and may lead to treated LLDPE's whose mechanical properties are unsatisfactory." Further, these treated LLDPE's are said to produce finished extruded articles which have a "non-uniform wall and a rough surface" as a result of "shark-skin" melt fracture. PCT '142 allegedly solves the difficulty by using thermo-mechanical treatment of the LLDPE in a molten state. The treatment involves introducing LLDPE having a density of 0.9 to 0.935 g/cm.sup.3 and a melt index over 3 dg/minute as a powder into a thermomechanical apparatus of an extruder while simultaneously introducing an organic peroxide at a level of over 0.05% and less than 1% (by weight of the polymer).
U.S. Pat. No. 4,598,128 (Randall et al.) describes ethylene polymer compositions being a blend of a first and second ethylene polymer. The second ethylene polymer is characterized by molecules having long chain Y-branches. Both polyethylenes can be made using the high pressure process (producing homopolymer low density polyethylene (LDPE)) or in a low pressure process (producing linear polyethylene having essentially no long chain branching). The blend can be prepared by using an extrusion process in which a portion of the polyethylene is irradiated and both the irradiated and non-irradiated polymers subsequently melt blended. The long chain Y-branched polymer is said to have a broad molecular weight distribution. The resultant blended composition is also said to have altered Theological properties without significantly increasing the molecular weight of the polymers. The compositions are said to be useful for coatings and production of shaped and molded articles (e.g., pipes, gas tanks and other molded auto parts).
While there have been several attempts at increasing the processability of linear heterogeneously branched polyethylene through use of irradiation, there continues to be a need for cost effective modification of polyethylene such that the resultant modified polymer is still useful for thermoplastic molding processes. In particular, there is a need for polyolefins having one or more improved processing characteristics such as higher zero shear viscosity, low high shear viscosities, improved melt flow (I.sub.10 /I.sub.2) properties, improved critical shear rate at onset of surface melt fracture, improved critical shear stress at onset of gross melt fracture, improved rheological processing index (PI), improved melt strength, higher green strength, greater filler/plasticizer/oil loading capabilities, and/or improved peroxide cure efficiency, while maintaining or improving physical properties such as tensile strength, impact strength, modulus of elasticity and relaxation time. In blown film processes high bubble stability, particularly combined with high polymer throughput, is a particularly desirable objective and in cast film and extrusion molding processes the ability to increase or maintain the polymer throughput rate and/or reduce or maintain extruder back pressure while improving draw down and/or reducing neck in is particularly desired.